the levo isomer of tetramisole. It is a broad spectrum anthelmintic which is used extensively as a veterinary drug for food producing animals. Metabolism and environmental studies necessitated the synthesis of 14C-labelled levamisole (6) and of its major metabolite (12). 14C-Tetramisole was obtained in two steps from 14C-thiourea. Resolution via salt formation and crystallization afforded 14C-levamisole